Friday, November 19, 2010

Local and remote effects of Sierra Nevada

The next two days offer a great opportunity to examine the influence of the Sierra Nevada on the meteorology of the Intermountain West.  As of 1430Z this morning, a high-amplitude upper-level ridge and surface anticyclone were planted over the Gulf of Alaska with a positively tilted upper-level trough off the Pacific coast of the contiguous US.  As a result, large-scale southwesterly flow extends across California, the Sierra Nevada, and the Intermountain West.

IR Sat/GFS Forecast Valid ~1430 UTC 19 Nov 2010
On the regional scale, the leading edge of 700-mb baroclinity is draped across central Nevada and northern Utah and is collocated with a sea level pressure trough that extends from the Sierra Nevada across the Great Salt Lake Basin.

IR Sat/RUC Valid 1500 UTC 19 Nov 2010
The trough lies within the so-called Great Basin cyclone region identified by Jeglum et al. (2010) as having a high frequency of Intermountain cyclone occurrence and genesis downstream of the High Sierra.  Thus, although the existence of frontal troughing and baroclinity is consistent with the large scale pattern, I suspect the Sierra Nevada are enhancing the trough and influencing the position and intensity of the baroclinity over Nevada.  See also Shafer and Steenburgh (2008) and West et al. (2010).  

This pattern will persist over the next 24-36 hours as the large-scale trough amplifies and digs off the Pacific coast.  Keep an eye on things and consider the role of the Sierra Nevada.  In addition to how that mountain barrier affects mass and momentum, consider how it alters the thermodynamic and moisture characteristics of airmasses and how this in turn might affect the frontal and precipitation dynamics over the Intermountain West.  

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